This invention relates to a kiln for dental ceramic work having a retort chamber in which dental ceramic material is heated to its burning temperature. More specifically, the invention is directed to a particular type of heating means for effecting the heating of the dental ceramic material.
Electrical resistance heating sources are used to heat the retort chamber of known dental ceramic kilns. Resistance wires composed of high-melting, oxidation free alloys are made to glow by the application of an electric current. The resistance heat source generally provides a radiation maximum lying in a wavelength range of about 6 to 8 .mu.m.
The kiln space within the known retort chambers is heated to the required temperature of about 900.degree. C. to 1100.degree. C. The ceramic material must be carefully heated by the resistance retort heating means in a uniform manner from the surface of the material to the interior thereof for a predetermined duration of burning time. Such a careful heating procedure is necessary to prevent the surface of the ceramic material from sintering before the ceramic mass has been freed of gaseous inclusions through the evacuation of the retort chamber through the use of a vacuous source. Such gaseous inclusions are incapable of penetrating through a sintered surface. Consequently, undesirable bubbles are formed which impair the aesthetic appearance of the finished product. The burning period of about 12 to 15 minutes is generally used in the prior art kilns to avoid the insufficient sintering of the interior of the ceramic material which is caused by convection delay. At the same time, the outer surface of the ceramic material is subject to the risk of overburning.
Use of the prior art resistance retort assemblies is particularly critical when ceramic masses are burned on to crowns and bridgework made of metal. That is, the adhesion between the metal and ceramic material still remains a point of risk, particularly when a nonferrous material is used. Very good results can only be achieved in these prior art retort assemblies by the careful coordination of the ceramic material to a specific metal or vice versa.